Re: Plaintext

Darrell Kindred (dkindred@cmu.edu)
Wed, 2 Jul 1997 16:58:46 -0400


Thompson, Christopher writes:
> Something's been bothering me for a few days.
>
> If we hadn't have known _anything_ about the plain text of the
> DES-encrypted message, we wouldn't have been able to decrypt it, even
> using brute force. Is this correct?

That's correct. If you have no way to distinguish the
correct decrypted message from random gibberish, then
brute-force keysearch won't help you. However, in most
cryptographic applications, you do know at least something
about the output you expect; you may know that it's English
text, that it probably contains no "control characters", or
that it has certain header fields.

The search is most efficient if you know exactly what you're
looking for, but even if you just have a moderately quick way
to separate the wheat from the chaff, brute-force key search
can do the job.

- Darrell